The bone morphogenetic proteins (BMPs) are members of a large, highly conserved, family of extracellular polypeptide signaling molecules related to transforming growth factor-.beta. (TGF-.beta.). There is now considerable evidence from expression studies, and from the in vivo effects of misexpression and mutations, that Bmp genes play key roles at many different stages of embryonic development, in both invertebrates and vertebrates (Kingsley et al., 1994, Dev. Biol. 166:112-122; Massague et al., 1994, Trends Cell Biol. 4:172-178; Hogan, 1995, Sem. Dev. Biol. 6:257-265). In the mouse, both spontaneous and induced mutations in a number of Bmp genes have shed light on their function in vivo. The first example to be described was a series of short ear mutations, which result from alterations in the Bmp5 gene (Green, 1968, J. Exp. Zool. 167:129-150; Kingsley et al., 1992, Cell 71:399-410; King et al., 1994, Dev. Biol. 166:112-122). Null mutants are viable, but have defects in cartilage development in specific parts of the skeletal system, as well as abnormalities in the lung, kidney and ureter in some genetic backgrounds.
Mutations in other Bmp genes have been generated by homologous recombination in embryonic stem cells. For example, Bmp7 homozygous null mutant mice die shortly after birth with major defects in eye, kidney and limb development (Dudley et al., 1995, Genes Dev. 9:2795-2807; Luo et al., 1995, Genes Dev. 9:2808-2820). Most Bmp4 homozygous mutant embryos die around the time of gastrulation and many exhibit a deficiency in extraembryonic and posterior/ventral mesoderm (Winnier et al., 1995, Genes Dev. 9:2105-2116), a finding consistent with the effect of BMP4 on mesoderm patterning in Xenopus embryos (Jones et al., 1992, Development 115:639-647; Graff et al., 1994, Cell 79:169-179; Harland, 1994, Proc. Natl. Acad. Sci. USA 91:10243-10246). Mutations have also been described in other members of the BMP superfamily, including mouse nodal, and Gdf5 (brachypodism) (Zhou et al., 1993, Nature 361:543-547; Conlon et al., 1994, Development 120:1919-1928; Storm et al., 1994, Nature 368:639-643).
Spermatogenesis takes places within the confines of the seminiferous tubules in the testis. A typical tubule is ensheathed by an outer basal lamina. Inside the lamina and attached to it is a layer of spermatogonial cells which continue to divide very slowly from puberty to late adult life. The self-renewing spermatogonial stem cell (known as an A.sub.O /A.sub.S cell) is very rare and it gives rise to other less primitive spermatogonial cells. These give rise to non-dividing spermatocytes which leave the basal layer and move towards the center of the tubule. These spermatocytes undergo meiosis and eventually give rise to mature sperm. The spermatogonia and differentiated derivatives are in intimate contact with the somatic Sertoli cells. Little is known about the growth factors/cytokines which regulate the proliferation of the spermatogonial stem cells, their differentiation into spermatocytes, the entry of the spermatocytes into meiosis, their differentiation into sperm and the way in which this whole complex process is co-ordinated in vivo. It has been difficult to obtain populations of cells having high concentrations of the most primitive spermatogonial stem cells in part because they form a very small proportion of the total spermatogenic cells of the testis. Additionally, cultures of spermatogonia generally only remain viable for short periods of time, that is, about 24-48 hours.